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        <title>css 响应式</title>
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        <h1>CSS 响应式</h1>
        <p>来源:<a href="http://www.ruanyifeng.com/blog/2017/05/css-variables.html?utm_source=tuicool&utm_medium=referral">http://www.ruanyifeng.com/blog/2017/05/css-variables.html?utm_source=tuicool&utm_medium=referral</a></p>
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        <p>蓝色</p>
        <div>绿色</div>
        <div id="alert">红色</div>
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